If "arbitrageur" means what you say it means, then I qualify under that category. I am a college student who buys and sells specific items online. I stick with one thing because it helps so you are educated and familiar with that one specific thing and when you are "hunting" for new products to buy for cheap, you already know about what they cost.
To answer your question, an ebay mini-business is a lot like opening your own mom-and-pop store, only you don't have rent fees. Depending on how hard you work, your work ethic, your customer service, etc. you could make a lot of money or you could barely break even. Customer service is worth more then making a killer profit on an item. Make enough to at least break even; don't be an *** and make people upset. The power belongs to the buyer--as a seller, you can have your "company" taken from you in a matter of 5 minutes if you piss off enough people.
Enter at your own risk. I'd do it as a hobby (like me) first, and if it grows good. Don't depend on it or you will soon find your self unemployed and no money coming in.